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SubjectRe: [RFC] respect the referenced bit of KVM guest pages?
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Hi

> Greetings,
>
> Jeff Dike found that many KVM pages are being refaulted in 2.6.29:
>
> "Lots of pages between discarded due to memory pressure only to be
> faulted back in soon after. These pages are nearly all stack pages.
> This is not consistent - sometimes there are relatively few such pages
> and they are spread out between processes."

I suprise this result really.

- Why this issue happened only on kvm?
- Why shrink_inactive_list() can't find pte young bit?
Is this really unused stack?

>
> The refaults can be drastically reduced by the following patch, which
> respects the referenced bit of all anonymous pages (including the KVM
> pages).
>
> However it risks reintroducing the problem addressed by commit 7e9cd4842
> (fix reclaim scalability problem by ignoring the referenced bit,
> mainly the pte young bit). I wonder if there are better solutions?
>
> Thanks,
> Fengguang
>
> ---
> mm/vmscan.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> --- linux.orig/mm/vmscan.c
> +++ linux/mm/vmscan.c
> @@ -1288,12 +1288,12 @@ static void shrink_active_list(unsigned
> * Identify referenced, file-backed active pages and
> * give them one more trip around the active list. So
> * that executable code get better chances to stay in
> - * memory under moderate memory pressure. Anon pages
> - * are not likely to be evicted by use-once streaming
> - * IO, plus JVM can create lots of anon VM_EXEC pages,
> - * so we ignore them here.
> + * memory under moderate memory pressure.
> + *
> + * Also protect anon pages: swapping could be costly,
> + * and KVM guest's referenced bit is helpful.
> */
> - if ((vm_flags & VM_EXEC) && !PageAnon(page)) {
> + if ((vm_flags & VM_EXEC) || PageAnon(page)) {
> list_add(&page->lru, &l_active);
> continue;
> }





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